Silicon-based optical communication devices, which function by use of optical fiber communication wavelengths of 1310 nm and 1550 nm that are used in a variety of systems such as a home-use optical fiber, a local area network (LAN), and so on, have great potential in terms of techniques that allow integration of optical function elements and electronic circuits on a silicon platform by use of a CMOS technique.
Recently, passive devices, such as a waveguide, an optical coupler, a wavelength filter, and so on which are silicon based, are studied very widely. Further, active elements, such as an optical modulator, an optical switch, and so on which are silicon based, can be listed as those that are important in the techniques used in the means for controlling optical signals used in the above communication systems; and these active elements are receiving great attention. The operation of each of an optical switch and a modulation element, which changes a refractive index by use of a thermooptic effect of silicon, is slow; thus, it can support a modulation frequency of up to 1 Mb/sec only, in terms of a device speed. Accordingly, an optical modulation element which uses an electro-optic effect is necessary, for realizing a high modulation frequency that is required in many optical communication systems.
Many of the presently suggested electro-optic modulators are devices, each of which uses a carrier plasma effect for changing density of free carriers in a silicon layer to thereby change a real part and an imaginary part of a refractive index, for changing the phase and/or the intensity of light. Pure silicon does not exhibit a linear electro-optic effect (Pockels), and change in a refractive index due to a Franz-Keldysh effect or a Kerr effect is very small; thus, the above effect is widely used. In a modulator which uses free carrier absorption, the output thereof is directly modulated in accordance with change in the degree of absorption of light propagating through Si. As a structure using change in a refractive index, a structure using a Mach-Zehnder interferometer is generally known, wherein it is possible to obtain an intensity modulation signal of light by causing interference relating to an optical phase difference between two arms to be occurred.
Free-carrier density in an electro-optic modulator can be changed by injecting, accumulating, excluding, or inverting free carriers. Many of those devices, that have been studied, have bad optical modulation efficiency, require the length on the order of millimeters for optical phase modulation, and require injection current density higher than 1 kA/cm3. For realizing miniaturization/high-integration, and for further realizing low electric power consumption, an element structure that has high optical modulation efficiency is required; and, by using the element structure, the length for optical phase modulation can be reduced. Also, in the case that the size of an element is large, the element is susceptible to temperature distribution on a silicon platform; so that there may be a case that an originally existed electro-optic effect may be cancelled out by the change in a refractive index of a silicon layer due to a thermooptic effect; and this may become a problem.
FIG. 25 shows a typical example of a silicon-based electro-optic phase modulator which uses a rib waveguide shape structure formed on an SOI substrate and is disclosed in each of Patent-related Document 1 and Non-patent-related Document 1. The electro-optic phase modulator is formed by applying p-doping and n-doping processes to slab regions extending laterally along both sides of a rib shape comprising an intrinsic semiconductor region. The above rib waveguide structure is formed by use of an Si layer on a silicon on insulator (SOI) substrate. The structure shown in FIG. 25 corresponds to a PIN-diode-type modulator which has a structure such that free carrier density in the intrinsic semiconductor region is changed by applying a forward bias and a reverse bias, for using a carrier plasma effect to thereby change the refractive index. In this example, an intrinsic semiconductor silicon layer 2501 is formed to include a p-type region 2504 which is in contact with a first electrode contacting layer 2506 and is doped at high concentration. In the figure, the intrinsic semiconductor silicon layer 2501 further comprises a region 2505 that is doped at high concentration by applying an n-type doping process, and a second electrode contacting layer 2506 connected to the region 2505. In the above PIN diode structure, it is possible to apply the doping process to each of the regions 2504 and 2505 in such a manner that the region exhibits carrier density of approximately 1020/cm3. Further, in the above PIN structure, the p-type region 2504 and the n-type region 2505 are positioned at both sides of the rib 2501, respectively, with a distance between the above two regions; and the rib 2501 is an intrinsic semiconductor layer. Further, FIG. 25 shows a supporting substrate 2503, a buried oxide film layer 2503, electrode wires 2507, and an oxide cladding 2508.
Regarding an optical conversion operation, it is connected to an electric power source in such a manner that a forward bias is applied to the PIN diode by use of the first and second electrode contact layers 2506 to thereby inject free carriers into the waveguide. At that time, the refractive index of the silicon layer 2501 changes due to increase of the free carriers; and, as a result, phase modulation of the light, which is transmitted through the waveguide, is induced. In this regard, the speed of the optical modulation operation is restricted by the life of free carriers in the rib 2501 and by carrier diffusion when the forward bias is removed. A PIN-diode-type phase modulator such as that of prior art explained above usually operates at a speed around 10-50 Mb/sec when it operates under application of a forward bias. On the other hand, although it is possible to increase an operating speed by introducing an impurity into a silicon layer for shortening the life time of the carriers, there is a problem that efficiency of the optical modulation is lowered by the introduced impurity. However, the most important factor that has effect on the operation speed relates to an RC time constant; and, in this case, the electrostatic capacity (C) at the time that the forward bias is applied becomes very large, as a result of large carrier life time of carriers in the PIN junction. Theoretically, it is possible to realize a high speed operation for the PN junction by applying a reverse bias; however, a relatively high driving voltage or an element having a relatively large size is required to obtain enough optical modulation amplitude.